In the last decade or so, vacuum arc evaporation has come into wide commercial use for deposition of metal, alloy, and metal compound coatings on a substrate to be coated. Vacuum arc discharges have also been used as ion sources for such applications as ion implantation, beam accelerators, and rocket propulsion.
The process of vacuum arc evaporation for coating or implanting a substrate includes a cathode target composed of the material to be deposited, and a substrate which is to be coated. The target is vaporized by a high current, low voltage arc plasma discharge in a vacuum chamber which has been evacuated to a pressure of typically less than 0.001 mbar. The substrates to be coated or implanted are usually placed in the vacuum chamber facing the evaporable surface of the target, at a distance of typically 10-100 cm. Typical arc currents range between 25 and 1000 amperes, with voltages between 15 and 50 volts.
The arc plasma discharge conducts electric current between a cathode and an anode through the plasma created by vaporization and ionization of the target material by the arc. The cathode (negative electrode) is an electrically isolated source structure which is at least partially consumed during the process. The consumable portion of the cathode is called the "target" and is of men fabricated as a replaceable element clamped to a cooled, non-consumable element called the cathode body. The anode (positive electrode) may be an electrically isolated structure within the vacuum chamber or may be the vacuum chamber itself, and is not consumed in the process.
An arc is ignited on the evaporable surface of the cathode target, commonly by means of mechanical contact, high voltage spark, or laser irradiation. The ensuing arc plasma discharge is highly localized in one or more mobile arc spots on the cathode target surface, but is distributed over a large area at the anode. The extremely high current density in the arc spot at the cathode, estimated to be 10.sup.6 -10.sup.8 amperes/cm.sup.2, results in local heating, evaporation, and ionization of the cathode source material.
Each arc spot emits a jet of plasma in a direction approximately perpendicular to the cathode target surface, forming a luminous plume extending into the region between the cathode and anode. The substrate to be coated or implanted is placed between or adjacent to the cathode and anode. The vapor of cathode material is typically further accelerated toward the substrate surface by an applied voltage, and condenses onto or becomes imbedded into the surface of the substrate. Reactive gasses may be introduced into the vacuum chamber during the evaporation process, resulting in the formation of material compounds involving the target material, reactive gas, and/or the substrate material.
Below about 70-100 amperes of arc current, depending on the target material, only a single arc spot exists on the surface of the cathode source material. At higher arc currents, multiple arc spots can exist simultaneously on the target surface, each carrying an equal fraction of the total arc current. An arc spot, in the absence of applied magnetic fields, tends to move randomly around the target surface, leaving a trail of microscopic crater-like features on the target surface.
An externally applied magnetic field exerts a force on the arc jet in a direction perpendicular to both the field lines and the jet, and can have a dominant influence on the large-scale average movement of the arc spot although the small-scale motion of the arc remains semi-random. The direction of the motion of the arc spot in a magnetic field is opposite or "retrograde" to the vector JxB direction expected based on ampere's law, considering the electron current emitted from the cathode. This phenomenon is due to complex dynamic effects within the arc jet, and has been widely reported and discussed.
An undesirable side effect of the vaporization of the target material at the arc spot is the generation of droplets of molten target material, which are ejected from the target by reaction forces due to expansion of the vapor jet. These droplets are commonly called macroparticles, and range in diameter from sub-micron to tens of microns. The macroparticles can become imbedded in the coating when they land on the substrate to be coated, forming objectionable irregularities, or the macroparticles can stick to the substrate and later fall off, causing pits in the coating.
Various strategies have been devised to reduce the number of macroparticles incorporated into the coating on the substrate. These strategies fall generally into two categories: (1) a first category using some form of magnetic field to control and accelerate the arc, thus reducing macroparticle generation, and (2) a second category using a filtering apparatus between the cathode source and the substrate so as to transmit the ionized fraction of the cathode output to the substrate, but to block the molten droplets.
The magnetic methods of the first category are generally simpler than the filtering methods, but do not completely eliminate macroparticle generation. The filtering methods of the second category are generally more effective at removing macroparticles than the magnetic methods, but require complex apparatus and reduce the source output significantly.
Filtering methods work by placing the substrate out of the line of sight of the cathode target surface, so that macroparticles emitted from the cathode do not impinge directly on the substrate. An angled filtering duct is interposed between the cathode and the substrate to transport the plasma to the substrate.
In order to reach the substrate, the charged plasma emitted from the cathode source is deflected electromagnetically within the filtering duct through an angle of 45.degree.-180' so as to pass through the bend in the filtering duct and to impinge on the substrate. The uncharged macroparticles are not deflected by the electromagnetic field and continue in a course which hits the walls of the filtering duct so that ideally the macroparticles do not reach the substrate. In practice, however, bouncing of macroparticles off the filter walls and/or entrainment of small particles in the plasma can result in transmission of some macroparticles through the filter to reach the substrate.
Prior filtered cathodic arcs have been based upon circular or cylindrical cathode and filter geometry, generally limiting potential applications to small substrates or special shapes.
Examples of the early work done in the field of arc evaporation are described in several United States patents, including U.S. Pat. No. 484,582 of Edison which describes the use of vacuum arc evaporation for depositing a coating onto a substrate; U.S. Pat. No. 2,972,695 of Wroe which describes a magnetically stabilized vacuum arc evaporation apparatus; U.S. Pat. Nos. 3,625,848 and 3,836,451 of Snaper which describe arc evaporation apparatus with particular electrode configurations, and the use of a magnetic field to increase the evaporation rate and to direct ions to the substrate; and U.S. Pat. Nos. 3,783,231 and 3,793,179 of Sablev, et al. which describe particular configurations of electrodes and shields, and describe use of a magnetic field activated whenever the arc spot moves off the desired evaporation surface of the cathode source material.
Examples of cathodic arcs confined within a circular or racetrack path upon the cathode are illustrated by U.S. Pat. Nos. 4,724,058 of Morrison; 4,673,477 of Ramalingam, et al.; and 4,849,088 of Veltrop, et al. Each of the foregoing references describe an arc evaporation apparatus using an arched magnetic field in the shape of a closed loop tunnel, which confines the arc spot to a closed loop racetrack trajectory at a fixed or movable location on the cathode surface. Confinement and acceleration of the arc by the magnetic field is said to reduce generation of macroparticles by the arc discharge. The means required to generate such a magnetic field are widely known in the art of planar magnetron sputtering. It is also known, for example, to move the electro-magnetic field generating means of the arc, either mechanically as taught by Ramalingam, et al. and by Veltrop, et al., or by use of multiple electromagnets as taught by Morrison.
Examples of elongated, cylindrical cathodes are included in U.S. Pat. Nos. 4,609,564 and 4,859,489 of Pinkhasov; 5,037,522 of Vergason; and 5,269,898 of Welty, all of which describe the use of an elongated cathode in the form of a cylinder or rod, and make use of the self-magnetic field of the arc current to force its motion along the length of the cathode. Welty teaches that macroparticle generation can be reduced by application of an additional axial magnetic field component to accelerate and control the arc motion.
U.S. Pat. No. 4,492,845 of Kljuchko, al. describes an arc evaporation apparatus using an annular cathode, and in which the evaporable cathode surface is its outer wall, facing a cylindrical anode of larger diameter and greater length than the cathode. The substrates to be coated are disposed inside the annular cathode, not facing the evaporable surface, and are coated by ionized material reflected back by the electromagnetic field at the anode. A coaxial magnetic field is described for enhancing the reflection from the anode. Macroparticles ejected from the cathode surface are not reflected electrically by the anode (although they may bounce off it mechanically). As a result, macroparticle incorporation in the coating is reduced.
Examples of efforts to reduce the number of macroparticles incorporated into the coating on the substrate by using some form of a filtering apparatus between the cathode source and the substrate to transmit the charged ionized fraction of the cathode output and to block the uncharged macroparticles are shown in work done by Aksenov/Axenov, Falabella and Sanders.
A publication by Aksenov, et al. ("Transport of plasma streams in a curvilinear plasma-optics system", Soviet Journal of Plasma Physics, 4(4), 1978) describes the use of a cylindrical plasma duct containing a 90 degree bend, with electromagnet coils to create a solenoidal magnetic field through the duct, and with a circular arc evaporation cathode at one end of the duct and a substrate at the other end. The plasma emitted by the cathode is reflected from the duct walls by the magnetic and electric fields present, and transported along the magnetic field through the duct to the substrate, while the uncharged macroparticles are not deflected by the magnetic or electrostatic fields and are intercepted by the duct walls.
U.S. Pat. No. 5,279,723 of Falabella et al. describes an apparatus essentially similar to the original Aksenov filter, using a cylindrical duct with a 45 degree bend, a circular or conical cathode and anode, and including improvements to various components including the shape of the cathode and the internal baffles which reduce macroparticle transmission.
U.S. Pat. No. 4,452,686 of Axenov et al. describes a straight cylindrical filtering duct with no bend, a circular cathode located at one end of the duct, electromagnet coils to generate a solenoidal magnetic field through the duct, and with an additional electrode located in the center of the duct which blocks direct line of sight deposition from the cathode to the substrate. Plasma emitted by the cathode is deflected by the magnetic and electric fields at the duct wall and central electrode, and transported along the magnetic field through the duct and around the central electrode. The uncharged macroparticles are not deflected by the magnetic or electric fields and are intercepted by the central electrode.
U.S. Pat. No. 5,282,944 of Sanders, et al. describes a device somewhat similar to that of U.S. Pat. No. 4,452,686 of Axenov, using a straight cyclindrical filtering duct and a central shield which prevents macroparticles emitted at low angles from the cathode from reaching the substrate directly. Electromagnet coils generate a magnetic field within the duct which is substantially solenoidal near the duct wall. The evaporable surface of the cathode in this case is the outer surface of a short cylinder oriented coaxially with the filter duct, such that the plasma emitted from the cathode is directed radially at the outer wall of the filter duct and is deflected through approximately 90 degrees by the magnetic field and the electric field at the duct walls, and transported along the magnetic field to the end of the duct at which the substrate is located. Internal electrodes are disclosed to enhance deflection of the plasma at the end of the circular filtering duct opposite to the end at which the substrate is located.
None of the references of the prior art disclose a cathode having an evaporable surface of rectangular shape and using magnetic field polarity reversal to control the movement of the arc on the cathode surface, nor is a filtering duct having rectangular cross section disclosed. Accordingly, despite the work illustrated above, there is still a need for an improved filtered cathodic arc. Preferably, the filtered cathodic arc would include a rectangular deposition source.
Rectangular deposition sources are desirable for the coating of large substrates, coating of sheet material in roll form, and for coating of continuous streams of smaller substrates on a linear conveyor or circular carousel. Development of rectangular planar magnetron sputtering cathodes in the 1970's has led to widespread commercialization of sputtering for the coating of substrates in such configurations (see, for example the magnetron sputtering cathode of Welty, U.S. Pat. Nos. 4,865,708 and 4,892,633).
Filtered cathodic arc sources have the advantage that the stream of vapor of cathode material emitted from the source is fully ionized, unlike non-arc-based deposition methods such as evaporation and sputtering. The fully ionized vapor stream from a rectangular source would allow greater control over the energy of the atoms arriving at the substrate for coating or implantation, and would increase the reactivity of the vapor in forming compounds with reactive gases in the system, or with the substrate directly.
The present invention realizes the benefits of a filtered cathodic arc (fully ionized vapor stream, elimination of splattered droplets) and the benefits of a rectangular source (uniform evaporation from the source and uniform deposition on the substrate using linear motion) in order to coat or implant a long or large substrate. It is a goal of the present invention, therefore, to provide a filtered cathodic arc on a rectangular vacuum-arc cathode to accomplish the tasks that cannot be accomplished by the prior art.